The modern athletic shoe is a highly refined combination of elements which cooperatively interact in an effort to minimize weight while maximizing comfort, cushioning, stability and durability. However, these goals are potentially in conflict with each other in that the efforts to achieve one of the objectives can have a deleterious affect on one or more of the others. As a result, the shoe industry continues in its efforts to optimize the competing qualities of cushioning, durability and stability.
In athletic shoes the sole ordinarily has a multi-layer construction comprised of an outsole, a midsole and an insole. The outsole is normally formed of a durable material to resist wearing of the sole during use. In many cases, the outsole includes lugs, cleats or other elements to enhance the traction afforded by the shoe. The midsole ordinarily forms the middle layer of the sole and is typically composed of a soft foam material to cushion the impact forces and pressure experienced by the foot during athletic activities. The foam midsole may be formed with or without the inclusion of other cushioning elements, such as a resilient inflated bladder. An insole layer is usually a thin padded member provided overtop of the midsole to enhance the comfort afforded to the wearer.
Up until about the 1970's, however, athletic shoes were by and large considered deficient in providing cushioning for the wearer's foot. Consequently, numerous injuries were sustained by those engaging in athletic activities. To overcome these shortcomings, over the ensuing years manufacturers focused their attention upon enhancing the cushioning provided by the athletic shoes. To this end, midsoles have over time been increased in thickness. These endeavors have further led to the incorporation of other cushioning elements within the midsoles (e.g., resilient inflated bladders) and other sole configurations intended to provide enhanced cushioning effects. The industry's focus on improving cushioning has resulted in a marked improvement of shoes in this regard. However, footwear stability has not always been so successfully addressed. In fact, the benefits realized in cushioning have sometimes led to a degradation of the shoe's stability.
To appreciate the potential harmful affects that could be attributed to instability, it is important to have a basic understanding of the dynamics of running. While the general population includes a wide variety of running styles, most people run in a heel-to-toe manner. However, in this running style the foot does not normally engage the ground in a simple back to front linear motion. When a person runs, the feet generally engage the ground under the approximate midline of their body, rather than to the sides as in walking (FIG. 15). As a result, the foot is tilted upon ground contact such that initial engagement with the ground (commonly referred to as the "rearfoot strike") usually occurs on the lateral rear comer of the heel. At rearfoot strike, then, the foot is ordinarily oriented with the big toe pointing upward and slightly outward. As the ground support phase progresses, the foot is lowered to the ground in a rotative motion such that the sole comes to be placed squarely against the ground. Inward rotation of the foot is known as eversion and, in particular, inward rotation of the calcaneus associated with articulation of the subtalar joint is known as rearfoot pronation. While, eversion is itself a natural action, excessive pronation, or an excessive rearward rate of pronation is sometimes associated with injuries among runners and other athletes.
Other running styles also include similar movements. For instance, runners who engage the foot at points other than the heel still tend to initially engage the ground along the lateral edge of the shoe. Thereafter, the foot similarly rotates inwardly so that the sole rests squarely on the ground. Some runners experience an outward rotation of the foot, known as inversion and supination, rather than the conventional inward rotation. Similar stability problems due to excessive rotations, or rate of rotation of the foot are sometimes attributed to the presence of varus and valgus conditions regarding the anatomy of the wearer (FIGS. 16-19). In general, varus and valgus conditions exist when the forefoot and/or midfoot including the metatarsals are oriented at an inclination with respect to the rearfoot such that the calcaneus is caused to pronate or supinate, respectively, when the sole of the wearer's foot is placed flush on the ground. Under these conditions, the foot pronates or supinates with every step and indeed even with standing, and can exacerbate the rotation under more demanding conditions such as exists in running. Lastly, rotative motions may be associated with lateral movements, as commonly seen in basketball.
In a natural barefoot condition a person's foot is normally provided with certain adaptations and mechanics which function to avoid the stability problems associated with rotative movements, irrespective of whether the rotation is due to running, lateral movements, or the presence of a varus/valgus condition. While the foot has the capacity to counter and stabilize supination of the foot, perhaps the more salient stabilizing qualities of the foot are directed to controlling pronation. In particular, a large number of the joints and connective tissues of the lower leg and foot cooperatively function to effect and control the foot's movements. The human "body" is able to process complex "information" and successfully use these natural mechanics to respond to performance demands relevant to stability requirements in fractions of a second. Moreover, through neuromuscular learning an individual is able to maximize the use of their anatomical endowment by anticipating upcoming events and responding accordingly.
The skeletal framework of the foot provides the requisite strength to support the weight of the body through wide ranges of activities. The foot is made up of 26 interconnected bones (FIGS. 11-14). While many of the joints between these bones are relatively inflexible due to the attachment of ligaments, a number of movable joints important to natural foot stability are present. The bones in the foot are commonly identified into three main groups: tarsus (the posterior group), metatarsus (the middle group) and phalanges (the anterior or distal group).
The foot is interconnected to the leg via the tarsus. More specifically, the tibia 10 and the fibula 11 (i.e., the leg bones) are movably attached to the talus 18 to form the ankle joint 15. In general, the leg bones 10, 11 form a mortise into which a portion of talus 18 is received to form a hinge-type joint which allows both dorsi flexion (upward movement) and plantar flexion (downward movement) of the foot. Talus 18 overlies and is movably interconnected to the calcaneus 19 (i.e., the heel bone) to form the subtalar joint 27. Subtalar joint 27 enables the foot to move in a generally rotative, side-to-side motion. Rearfoot pronation and supination of the foot is generally associated with movement about this joint. Along with talus 18 and calcaneus 19, the tarsus further includes navicular 20, cuboid 21 and the outer, middle and inner cuneiforms 22-24. The four latter bones 21-24 facilitate interconnection of the tarsus to the metatarsus.
The metatarsus is comprised of metatarsals 31-35. Metatarsals 31-35 are relatively long bones which extend forwardly across the middle part of the foot to interconnect the tarsus and the phalanges. Each of the metatarsals are aligned with and connect to one of the phalanges. For example, the first metatarsal 31 is connected to the hallux 40 (i.e., the big toe), whereas the fifth metatarsal 35 is connected to the fifth or smallest digit 44. The first, second and third metatarsals 31-33 are attached on their proximal ends to the outer, middle and inner cuneiforms 22-24, respectively. The fourth and fifth metatarsals 34, 35 are both connected to cuboid 21.
The phalanges are the bones which associated with the toes. The phalanges include 14 bones altogether. The hallux 40 (i.e., the big toe) includes the distal phalange of the hallux 40a and the proximal phalange of the hallux 40b. The remaining phalanges of the second to fifth digits 41-44 (i.e., the small toes) are each comprised of a distal phalanx 41a, 42a, 43a, 44a, a middle phalanx 41b, 42b, 43b, 44b, and a proximal phalanx 41c, 42c, 43c, 44c. The phalanges of toes 40-44, and especially hallux 40, are hingedly attached to the metatarsals for significant movement. As discussed below, these movements can play an integral role in controlling eversion and inversion of the foot. As a practical matter, the hallux is by far the prominent toe with respect to supporting weight, providing propulsive force and stabilizing eversion of the foot.
The muscles in the foot are interconnected with the bones of the foot to impart the desired motions (FIGS. 20-24). The muscles having the primary responsibility for controlling eversion are the tibialis posterior 52, flexor digitorum longus 53, flexor hallucis longus 54, extensor hallucis longus 55 and tibialis anterior 56. All of these muscles 52-56 are large and powerful muscles which originate in the lower leg and attach to various bones in the foot. The peroneus longus 57, peroneus brevis 58 and extensor digitorum longus 59 for toes 41-44 perform as evertors of the foot and aid in controlling inversion.
Tibialis posterior 52 is attached to the posterior of tibia 10 and to the plantar side of navicular 20 and cuneiforms 22-24 (FIGS. 20, 23 and 24). The tendon associated with muscle 52 wraps around a groove in the medial malleolus portion of tibia 10 which functions as a fulcrum point to enable the muscle to impart force the navicular and cuneiforms. The action of muscle 52 enables it to counter rearfoot pronation of the foot.
Flexor digitorum longus 53 also originates along the posterior of tibia 10 (FIGS. 20-22). This muscle is connected on its distal end to the plantar sides of the distal phalanges of each of the second through fifth digits. The tendon associated with muscle 53 is also wrapped about a medial malleolus tibia 15 portion of which acts as a fulcrum point to enable force to be applied the plantar side of toes 41-44. This muscle functions to collectively plantar flex (i.e., bend or curl downward) the small toes of the foot.
The flexor hallucis longus 54 and extensor hallucis longus 55 are both attached to hallux 40 to facilitate plantar flexion (i.e. downward bending) and dorsi flexion (i.e., upward movement) thereof, respectively (FIGS. 20-21). In particular, flexor hallucis longus 54 is attached to the posterior of fibula 11 and to the plantar side of hallux 40. The tendon associated with muscle wraps around a medial groove in talus 18 and calcaneus 19 which functions as a fulcrum point to direct force to the plantar side of hallux 40. Extensor hallucis longus 56 connects to an anterior part of the tibia 10 and along the dorsal side of hallux 40. The connection of these two powerful muscles to only the hallux provides a considerable range of movement and strength to the digit.
Tibias anterior 55 attaches to an anterior portion of tibia 10 and along the first metatarsal 31 (FIG. 20). Muscle 55 functions to counter eversion and enable inversion of the foot. Also, by connecting to first metatarsal 31, the muscle additionally increases the strength of movement of hallux 40.
While a multiplicity of intrinsic muscles in the foot are also involved in causing and controlling eversion and inversion, they are much smaller and hence have a diminished role in comparison to the above-discussed muscles. A full discussion of these muscles has therefore been omitted.
As can be readily appreciated, the muscles associated with plantar flexion of the toes roughly divide the stability operations of the foot into two parts. One part is comprised of the strong, dominant hallux 40 which is independently moved and controlled, e.g., by a pair of large, powerful muscles 54, 55. The other part is comprised of the lesser but still important remaining digits 41-44 which are collectively controlled for movement independent of hallux 40, e.g., by muscle 53 and an extensor muscle (not shown). The independent and strong movements of these two parts are important contributors to an individual's ability to control eversion and inversion.
The shoe industry's focus on improving cushioning has led to the use of thickened midsole elements. Increased thicknesses in the midsole, while accomplishing its purpose of enhancing cushioning, have also tended to make soles increasingly inflexible. This reduced flexibility in the sole can substantially limit the ability of hallux 40 and other toes 41-44 to perform their natural stabilizing movements. Moreover, the inflexibility of the sole inhibits the ability of hallux 40 to act independently of the other toes, which further diminishes the ability of the foot to stabilize itself. In many of today's shoes the toes have a limited freedom of motion and are forced to move in an essentially monolithic manner. Increased demands for resistance to excessive eversion or inversion can be placed upon muscles which have tendons inserting to the middle portions of the foot, such as the tibialis posterior 52, tibialis anterior 56, and peroneus longus 57. These structures can thereby be overloaded. As a result, the combined effects of a relatively inflexible outsole and a thick midsole can reduce the foot's ability to respond and control eversion, inversion and other rotative motions.
Further, the use of a thickened midsole raises the foot to a higher level above the ground as well as forming a less flexible member. This combination of features exacerbates potential stability problems. More specifically, during heel strike, the sole initially engages the ground along the rear lateral portion of the sole. Due to the relative relative stiffness in compression of the soles of conventional footwear, this contact functions as a fulcrum point about which a lever arm is created as the foot rotates as it is lowered to the ground. As a result the conventional sole is not able to match the lesser rotation which occurs in the natural barefoot condition. As can be appreciated, the combined effects of raising the foot higher above the ground, using a relatively inflexible sole, and the creation of an extended lever arm can cause the shoe to rotate medially, at a faster rate and to a greater degree than would otherwise be experienced. The possible detrimental affects of pronation can be furthered as a result of the foot's reduced capacity to stabilize such motion through active use of the toes.
The reduced flexibility of the sole can also hinder the ability of the foot to attain a powerful and smooth propulsive movement. In an effort to offset this shortcoming, many manufacturers have incorporated a feature known as "toe spring" into their soles. The term "toe spring" is a misnomer, however, since it does not involve any springing of the toes. Instead, toe spring merely refers to the upward rounding of the front end of the sole, to enable the sole to roll off the ground in a smoother fashion during the propulsive portion of the ground support phase of the step. This construction finds its historical origin, e.g., in wooden dutch shoes which embody the ultimate in inflexible soles. While the introduction of toe spring can achieve a smoother roll off for the modern shoe, this upward rounding of the sole can further reduce the ability of the wearer to utilize the toes effectively in stabilizing the foot and in effecting propulsion, in particular with respect to high speed movements, ballistic jumping, or when rapid lateral movements are required.
The problems associated with a thickened sole were alleviated to some extent by the use of a V-shaped groove construction as disclosed in U.S. Pat. No. 4,562,651 to Frederick et al. The increased flexibility provided by these grooves permitted the wearer to more easily dorsi flex the toes but offered little in the way of permitting plantar flexion of the toes. Moreover, the V-groove construction did not alter the generally monolithic movement of the toes, or facilitate independent flexion of the toes. Hence, the stability and performance of athletic footwear in this regard was not dramatically enhanced.
A Tiger marathon shoe of the 1950's partitioned the entire toe box into two encapsulated compartments. The big toe was inserted into the medial compartment, whereas the smaller toes were received into the lateral compartment. Each compartment was enclosed on all sides by the upper and sole such that a complete slot extends rearwardly in the shoe between the compartments. Although this construction would readily increase the independent movement of the big toe, the insertion of two layers of upper material as well as the double row of stitching and/or adhesive used to attach a conventional upper to a conventional sole between the wearer's toes could cause toes to experience considerable chafing.
U.S. Pat. No. 3,967,390 to Anfruns also discloses a shoe provided with compartments in the toe box. While Anfruns asserts that the construction enhances independent movement of the toes, the problems of chafing and frictional resistance discussed with respect to the Tiger shoe could potentially be magnified four fold with the additional slots defined in this construction. Indeed, due to the natural closer spacing found between the smaller toes the discomfort of this shoe would far exceed that of the Tiger shoe.
U.S. Pat. Nos. 4,837,949 and 5,024,007 to Dufour, German Patent No. 680,698 to Thomsen, and PCT Application No. WO 9105491 to Ellis each utilize some form of longitudinal and transverse grooves or hinges in its sole structure. However, none of these soles locate the grooves in locations that optimize the use of the structures of the foot, so that it can substantially function in a manner more consistent with the natural barefoot condition during athletic activities.
A limited amount of independent toe movement is afforded by those shoe uppers which provide sufficient clearance in the toe box to permit the toes to "scrunch up" or plantar flex inside of the shoe. Although this limited movement of the toes provides an incremental enhancement of stability, it falls far short of the performance afforded by the present invention. Also, in order to accommodate this sort of limited toe movement, a snug and comfortable fit of the upper about the foot is often sacrificed. The looser fit not only reduces running efficiency and comfort, but also tends to result in the formation of blisters or other irritations, in particular, during explosive lateral movements such as encountered in basketball and other similar activities.